Fluid loss additive

ABSTRACT

An additive for aqueous fluids including aqueous acid solutions, brines, fracturing fluids, work over fluids, etc., and a method for treating a heterogenous permeable subterranean formation with such an aqueous fluid containing said additive is taught. The additive comprises a mixture of particulate oil soluble resins one being friable and the other pliable.

United States Patent I 1 I 1 91,566

Crowe 1 June 24, 1975 FLUID LOSS ADDITIVE 3.3l9,7l6 5/;967 Dill l66/2823,455,390 7/l969 Gallus 252/855 X U Cums Tulsa1okla- 3,711,405 i g973 Pe ct al 252/355 73 Assignee: The Dow Chemica' Company, 3.724 549 4/l973Dill .2 [66/282 Midland, Mich.

[221 Filed: Sept 1973 Primary Examiner-Herbert B. Guynn [21] Appl. No.:399,214 Attorney, Agent, or Firm-Bruce M. Kanuch Related US. ApplicationData [60] Division of Ser. No. 249.085, is a continuation-in-part ofSer. No. 177,680 Sept. 3. 1971. abandoned. [57] ABSTRACT [52] US. Cl252/8.55 R; l66/282; 166/283;

260/296 NR; 260/296 RB; 260/296 ME; An addmve for aqueous fluidsincluding aqueous acid 260/33 4 R solutions, brines, fracturing fluids,work over fluids, 511 Int. Cl E21b 43/25 and a men! "eating a hetemgemusP [58] Field of Search 252 855 R, 8.55 c; subtemnea" fmmafim' with suchaquwus 166 2 2 2 3 295 307- 2601334 R 29 ME fluid containing aidadditive iS taught. The additive comprises a mixture of particulate oilsoluble resins 56] Rehnnces cued one being ffiaifle and the otherpliable.

UNITED STATES PATENTS 3.302.719 2/1967 Fischer 166/205 X i Claim, 2Drawing Figures PATENTED J UN 24 I975 SHEET SHEET PATENTEDJUN 24 1915Obs QQM

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FLUID LOSS ADDITIVE CROSS REFERENCE TO RELATED APPLICATIONS BACKGROUNDOF THE INVENTION The problem of decreasing the permeability of certainportions of a subterranean formation so that certain treatments thereofwith aqueous fluids can be made more efficient is well known in the art.For exam ple, when acidizing a heterogenous permeable formation, it isimportant to contact the less permeable zones with the acid. Manyadditives, generally known as diverting agents have been developed forthis use. Also, when fracturing formations, more efficient treatmentscan be achieved if'the fracturing fluid is not lost to the formation butinstead can be further utilized to increase the length and width of thefractures. In these instances fluid loss additives are desirable. Thereare also other treatments where fluid loss control is desired.

Many materials have been employed as fluid loss control agents, and asdiverting agents, e.g., see the teachings of U.S. Pat. Nos. 3,319,716;3,281,354; 3,417,817 and the like. However, the additive of the presentinvention can be employed as a diverting and /or fluid loss agent andpossesses certain advantages not possessed by others. First, it isstable in acid solutions. The additive can be easily removed from theformation by contacting it with a liquid hydrocarbon, e.g., crude oil,thus preventing residual plugging and formation damage. In addition,only a very small quantity of SUMMARY OF THE INVENTION The principles ofthe present invention may be applied to improving hydrocarbon recoveryfrom heterogenous productive zones of underground formations. It mayalso be applied to generally improving the permeability of subterraneanformations for improved fluid flow therethrough.

The additive comprises a mixture of at least two oil soluble particulateresins. One resin remains hard and friable when dispersed in an aqueousmedium. The second resin is a material which is soft and pliable whendispersed in an aqueous medium.

Pliable means that a one inch square cube of a speciflc resin can becompressed by at least about onetenth its height without producingstress fractures in the cube. Friable means that stress fractures willoccur in a one inch cube of a specific material when compressed by aboutone-tenth, or less, of its original height. W

The resins may be derived from natural occurring materials, or they maybe synthetic, meaning they are prepared from the union of a large numberof molecules of one or; more compounds, or they are derived fromchemically modified natural resins.

The particle size of the friable resin is less than about 300 micronsand greater than about 0.5 micron. The particles of the pliable resinrange in size from about 0.05 to about 30 microns,

BRIEF DESCRIPTION OF THE DRAWING The FIGURES illustrate a plot of thedata generated in Example 2.

DETAILED DESCRIPTION OF THE lNVENTlON One of the oil soluble resins isfriable whendispersed in an aqueous medium, Resins which may be employedhaving these characteristics include,- for: example, oil solublepolystyrenes, terpenes, phenolics (substituted in the ortho or parapositions), modified alkyd resins, styrene-vinyl toluene copolymers,dimerized rosin acids, esters of dimerized rosin acids, acrylics andother such resins.

The'other oil soluble resin is pliable when dispersed in an aqueousmedium. Resins which may be employed having these characteristicsinclude, for example, certain latices such as aqueous dispersions ofhighmolecular weight thermoplastic resins such as, for example, ethyleneinterpolymers (such as sold under the'trade mark Elvax by E. 1. DuPontDe Nemours & Co., Inc.), ethylene-vinyl acetate resins, ethylene-vinylacetatewaxy dispersions, polyisobutylene resin dispersions (such as soldunder the trade name Oppanol B Dispersion) and other high molecularweight resins such as ethylene-acrylate resins, ethyl-ethylacrylateresins, ethyl-vinyl acetate, and the like.

The weight proportion of the two resins in the mix ture has not beenfound to be critical. Weight proportions ranging from 20:l to 1:20(friable to pliable resin) are operable.

In the use of the fluid loss additive, the ingredients may be added to atreatment fluid separately or they can be premixed, with or without anadditional fluid, and employed as needed. For example, to aid indispersing the oil soluble resins into an aqueous treatment fluid, themixture can be premixed with a small amount of an alcohol (mono or polyhydric) e.g., glycerol, etc., and/or a surfactant to render the resinmixture water wettable. Likewise, a surfactant may be added to theaqueous treatment fluid or the premix to aid in uniformly dispersing theresin particles into the treatment fluid.

An especially useful embodiment of the invention is a premix formed bydispersing a mixture of such resins into a mono and/or polyhydricalcohol to form a dispersion which is then easily mixed with varioustreatment fluids. Water, surfactants, antifoaming agents and the likecan also be included in the premix composition.

In plugging permeable portions or zones of a heterogenous permeableformation the ingredients or a premix are added uniformly to an aqueoustreatment fluid. The composition of the invention may also be added as aconcentrated slugto the aqueous treatment fluid or the formation may becontacted with an aqueous dispersion of the mixture and then with atreatment fluid.

Whenthe mixture is employed as a diverting agent it is provided in atreatment fluid in an amount which is effective to divert at least aportion of the treatment .fluid from a more permeable'portion of theformation to a less permeable portion. When employed as a divertingagent, for example, acid treatment fluids, the mixture (total amount ofboth resins) is incorporated into the treatment fluid in an amount of atleast one half pound per 1,000 gallons of treatment fluid. The upperlimit is dependent on the efficiency of the particular system beingemployed. The exact amount employed can, however, be varied over a widerange. It has been discovered that in general greater amounts should beemployed as the formation temperature increases.

When employed as a fluid loss additive generally at least about 2 pounds(total weight of both resins) per 1,000 gallons are employed.

in general, a sufficient amount of the resin mixture should be added tothe treatment fluid so that the fluid loss of an aqueous dispersion ofthe mixture through a one inch diameter by one inch thick Bereasandstone core (having an initial permeability of from about 50-200 md.)conducted in a high pressure cell at 1,000 psi and 150F is less thanabout 25 ml in 25 minutes.

The fluid loss additive is effective at temperatures up to the meltingpoint temperature of the lowest melting resin in the additive.

The additive can be employed in various aqueous treatment fluidsincluding acidizing fluids, e.g., HCl, HF, hydrobromic, organic acids,etc., aqueous based fracturing fluids, work over fluids, e.g., brine,water,

was first evaluated by means of fluid loss tests using a Bariod HighPressure-High Temperature test cell moditied to accept cores. Tests werecarried out by flowing fluids through Berea sandstone cores (200 ml.)having a surface area of 5.35 cm. and a thickness of 2.54 cm. Nitrogengas at a pressure of l,000 p.s.i. was used to force the fluid throughthe cores. The tests were conducted at a temperature of about l50F.Total fluid flow through the core was recorded at time intervals of l,4, 9, l6 and 25 minutes. Efficiency of the diverter was judged on thebasis of ability to retard flow. The following Table I shows the effectof various resins and resin combinations upon acid flow through thecore. The carrier fluid consisted of 15% HCl aqueous solution containing0.4% by weight of hexynol and 0.2% of a surfactant.

The pliable resin comprised a high molecular weight ethylene-vinylacetate aqueous dispersion containing about 50 per cent by weight ofsolids. The per cent shown in the Table I for the pliable resin is theper cent by weight of solids in the dispersion. The particles in thedispersion have a size ranging from about 0.4 to 0.6 micron. The friableresin with a high molecular weight hydrocarbon resin having a maximumparticle size of about microns with about 70-77 per cent being smallerthan about 2 microns. The results are shown in the following Table I.

TABLE I Additive-Per Cent by Weight Based on Total Weight of Fluid LossMin. Test No. Treatment Fluid l 4 9 I6 25 l None 100 ml in 20 sec.

2 0.2 pliable resin 3.0 3.4 3.8 4.0 4.3

0.24 friable 3 0.24 friable 100 4 2.4 friable 87 5 0.2 pliable l00 6 2.0pliable 63 79 88 96 7 0.2 pliable 2l 69 89 2.4 friable 8 2.0 pliable 6I3 25 39 72 .12 friable 9 0.2 pliable 4.5 4.9 5.3 5.7 6.0

.l2 friable l0 0.2 pllable 2.8 3.2 3.6 3.9 4.4

0.36 friable ll 02 pliable 2.5 3.0 3.4 3.8 4.2

0.48 friable and the like. The aqueous treatment fluid should besubstantially free of hydrocarbon solvents, e.g., oils, for the resinsemployed.

One specific application of the additive is in fluids, e.g., acidizing,which must penetrate a gravel pack to be most effective. In thisparticular use it is desirable for the treatment fluid and additive topenetrate the gravel pack so that the formation can be effectivelytreated. The resin particles in the mixture are provided in asufficiently small size that at least about 75 per cent of the particlepass through the specific gravel pack involved. For example, it has beendetermined that at least about 75 per cent of particles having a maximumsize of about 44 microns will pass through a gravel pack composed of 20+40 mesh (U.S. Standard Sieve Series) sand.

EXAMPLE I The efflciency of various resins and resin mixtures it isshown by these tests, that an obvious synergistic effect is present whenboth resins are employed. The combination of resins is much moreeffective than either resin alone, even when a single resin is used in amuch larger quantity. It is also shown that the synergistic effect ispresent even when the weight ratio of resins is varied over a widerange.

EXAMPLE II acid was forced through the cores by means of 100 p.s.i.nitrogen pressure. Acid was allowed to flow through both cores untilsteady flow rates were established. The reservoir was then emptied andrefilled with per cent hydrochloric acid containing a concentrationequivalent to l lb. of finely ground friable oil soluble synthetichydrocarbon resin and 1 quart of a pliable ethylene-vinyl acetate resindispersion (50% solids) per 1,000 gal. of acid. These resins are fullydescribed in the previous Example I. This acid mixture was then forcedthrough the cores at the same I00 p.s.i. pressure. Flow rates weremeasured and acid volumes recorded. From this and the previous datausing the acid alone, a plot was prepared (FIG. 1) of apparentpermeability versus total acid volume. The dotted lines represent flowof 15 per cent hydrochloric acid through the two cores while the solidline represents flow of the same acid containing the diverter of thepresent invention. The normal flow of acid through the more permeablecore is about ten times greater than for the less permeable core. Uponintroduction of the acid containing the diverter, the flow rates arequickly equalized. The proportion of acid flowing through the coresduring the initial phase of the test was 9| per cent of the acid flowingthrough the more permeable core while only 9 per cent enters the lesspermeable core. During the period when the acid-diverter mixture isbeing used, however, 55 per cent of the acid entered the highpermeability core while 45 per cent entered the less permeable core.

Following the above procedure the ability of the agent to besuccessfully removed from the formation is illustrated in FIG. 2. Afterthe core had been treated by the above procedure. purified kerosene wasrun through the plugged cores and as is shown in FIG. 2, the originalpermeability of the core was restored. Thus no formation damageoccurred.

EXAMPLE Ill To illustrate the successful clean-up ability of the presentinvention the following test was run.

A [5 per cent hydrochloric acid was forced through a l X 3 inch Bereasandstone core, held in a Hassler sleeve, until a constant flow rate wasobtained. lnjection pressure was 100 p.s.i. and the test temperature was75F. Once the initial permeability of the core was determined the sameacid containing a certain fluid loss additive was injected under thesame test conditions. In each instance the core became almost completelyplugged.

Following the above treatment purified kerosene was injected through thecore at the same temperature and pressure, until a constant flow ratewas stabilized and the permeability of the core was calculated.

Three HCl solutions were tested each containing a different additive.The first contained about 0.2 per cent by weight of hexynol, 0.4 percent of a pliable resin composed of a high molecular weightethylenevinyl acetate resin and 0.24 per cent of a friable hydrocarbonresin. The second HCl dispersion contained the same constituents as thefirst except that the 0.4 per cent pliable resin was replaced with 0.24per cent of gum karaya. The third HCl acid dispersion was identical tothe first except that the 0.24 per cent friable resin was replaced with0.24 per cent of finely ground (-325 mesh) silica flour.

in the above test the permeability of the core plugged with the firstacid dispersion (practice of the present invention) was restoredcompletely to its original value. The permeability of both of the secondand third were restored to less than about 87 per cent of their originalvalue, thus evidencing one of the unexpected properties of the presentinvention.

What is claimed is:

l. A low fluid loss additive dispersible in aqueous fluids employed fortreating subterranean formations comprising:

a. a liquid mono or polyhydric alcohol and a hetero geneous mixture ofdiscrete particles consisting essentially of;

b. a particulate oil soluble pliable resin ranging in size from about0.05 to about 30 microns dispersed in said alcohol; and

c. a particulate oil soluble friable resin ranging in size from about0.5 to about 300 microns dispersed in said alcohol, said friable andpliable particles being present in a weight ratio to each other rangingfrom about 1:20 to 20:1 and said composition being essentially free ofan oil in which said particles are soluble.

I *0 i i

1. A LOW FLUID LOSS ADDITIVE DISPERSIBLE IN AQUEOUS FLUIDS EMPLOYED FORTREATNG SUBTERRANEAN FORMATIONS COMPRISING: A. A LIQUID MONO ORPOLYHYDRIC ALCOHOL AND A HETEROGENEOUS MIXTURE OF DISCRETE PARTICLESCONSISTING ESSENTIALLY OF: B. A PARTICULATE OIL SOLUBLE PLIABLE RESINRANGING IN SIZE FROM ABOUT 0.05 TO ABOUT 30 MICRONS DISPERSED IN SAIDALCOHOL; AND C. A PARTICULATE OIL SOLUBLE FRIABLE RESIN RANGING IN SIZEFROM ABOUT 0.5 TO ABOUT 300 MICRONS DISPERSED IN SAID ALCOHOL, SAIDFRIABLE AND PLIABLE PARTICLES BEING PRESENT IN A WEIGHT RATIO TO EACHOTHER RANGING FROM ABOUT 1:20 TO 20:1 AND SAID COMPOSITION BEINGESSENTIALLY FREE OF AN OIL IN WHICH SAID PARTICLES ARE SOLUBLE.